Spark ignition engines that employ single or multipoint fuel metering concepts rely upon a throttle body to proportion the air flow rate in response to the operators commanded power level. Simply put, the more air flow allowed into the engine, the higher the power output. For most engines, the operator commands power via the foot pedal which either directly moves the throttle plate through an intermediate cable, or indirectly with a drive-by-wire throttle. In either case, the fuel management system adds the correct amount of fuel as required by the commanded airflow to achieve a desired air-to-fuel ratio (AFR).
As an engine is driven though different speeds and loads, the fuel management system calculates and delivers the ideal amount of fuel in proportion to the airflow to both maximize performance and minimize emissions. Of course, control of the air-fuel ratio is an important part of engine performance and emissions minimization. However, there are several common problems exhibited by internal combustion engines that tend to disturb the AFR.
In an ideal situation, the same ratio of air and fuel are introduced into each cylinder and in a mixed form, or in other words, in the form of a homogeneous charge. This results in a power balanced engine with excellent emission characteristics. In the real world, air flow balance from cylinder to cylinder is achieved only with significant intake system development. Intake system development is extensive for automotive engines utilizing multipoint injection, but intake systems on medium and heavy duty engines are generally neglected. This is especially true for diesel engines because diesel engines operate with excess air in the combustion chamber, so small variations in the air flow from cylinder to cylinder are not critical to achieving good emissions. However, when a diesel engine is converted to a gaseous fuel, the intake system becomes more important.
On the fuel side of the equation, carburetors have provided limited performance and can meet older emission standards, however it should be noted that no carbureted engines are currently sold in the United States, as they simply will not pass current emission standards. Next is the single-point metering concept, where typically two fuel injectors are mounted in a throttle body and perform basically like an electronic carburetor. Again, this type of a system has difficulty meeting current emissions levels. The current state of the art is the multipoint injection system where one injector is used per cylinder. When coupled with an engine with an intake system that has been air flow balanced, the system is capable of meeting the most stringent emissions standards.
Unfortunately, the multipoint system which has worked so well for gasoline engines is not a good solution for a diesel engine converted to natural gas. Therefore, there remains a need to overcome one or more of the limitations in the above-described, existing art.
It will be recognized that some or all of the Figures are schematic representations for purposes of illustration and do not necessarily depict the actual relative sizes or locations of the elements shown. The Figures are provided for the purpose of illustrating one or more embodiments of the invention with the explicit understanding that they will not be used to limit the scope or the meaning of the claims.